The field of the present invention is jet reaction thrust control of attitude (pitch, yaw and roll) of a moving body. More particularly, the present invention relates to jet reaction thrust control of a vehicle wherein successively oppositely directed and selectively variable impulse bits are applied to the vehicle. The vehicle comprises a source of pressurized motive gas communicating to pairs of attitude control nozzles which are oppositely disposed with respect to a control axis, and bistable valving controlling flow of the motive gas to the nozzles. The magnitude of the impulse bits applied to the vehicle is the time integral of thrust, and duration of thrust for each pulse. The oppositely directed thrust pulses from the attitude control nozzles are changed stepwise between near-zero, and a determined level. Because the impulse bits are applied to the vehicle in successively opposite directions, the vehicle is forced into an attitude oscillation. This attitude oscillation is known to those in the art as a limit cycle, and the control method is known as a Pulse Duration Modulation (PDM) or bang-bang system. In applications where the impulse bits are sufficiently small in comparison to the vehicle mass and are applied at a frequency above the oscillation threshold frequency of the vehicle, the latter responds only to the net average force without oscillation.
A conventional PDM vehicle control system utilizing bistable valve structure is known in accord with U.S. Pat. No. 3,278,140, issued Oct. 11, 1966 to K. C. Evans. According to the teaching of Evans, one of three bistable valves is utilized to control each one of the pitch, yaw, and roll axis of a vehicle. Each of the bistable valves comprises a body defining a nozzle directing a motive gas stream toward a splitter which defines a bifurcated flow path. Control ports are located on each side of the fluid stream as it travels toward the splitter. The chamber through which the fluid stream travels is configured to cause a vacuum in the control ports in response to flow of the fluid stream. Consequently, closing one of the control ports to atmosphere while leaving the opposite port open causes rarification switching of the fluid stream toward the closed port.
With a control scheme as taught by Evans, a vehicle must comprise at least three bistable valve apparatus to control the attitude of the vehicle in each one of the pitch, yaw, and roll axis. Consequently, the control apparatus may comprise a considerable portion of the total weight of a small vehicle. When it is desired to make a vehicle which is man-portable, or which can be carried upon other small, light-weight vehicles, the total weight of the vehicle is a critical design parameter. In such circumstances, the weight of a control device as taught by Evans may be a prohibitively large portion of the permissible vehicle weight. Of course, if the control scheme according to Evans is utilized nevertheless, the performance of the vehicle may fall short of that required.
Another aspect of the Evans control scheme which is not entirely satisfactory is the use of rarification switching of the fluid stream between its two positions within the bistable valve. Such a switching arrangement can result in the PDM characteristic of the control system being variable and dependent upon atmospheric pressure level. As a result, a vehicle which operates satisfactorily near sea level may have an unsatisfactorily slow PDM rate when used at a higher altitude.
Still another undesirable aspect of the Evans teaching when applied to small vehicles is the use of three separate motive fluid streams to effect control in the three control axes. Because each of the fluid streams must be flowing so long as control of the vehicle is to be effected, a considerable portion of vehicle total energy may be dissipated by the three streams in combination.